Carbon-carbon composites are a class of unique materials whose properties, especially at elevated temperatures, make them attractive for various aerospace applications. The materials are composites, although often all the composite elements are comprised essentially of carbon in its various allotropic forms. Factors such as the degree of graphitization, purity, pore structure, specific surface areas, surface complexes, oxygen availability, and temperature have a strong influence on the oxidation of carbon materials. Oxidation of carbon-carbon composites at elevated temperatures in an atmospheric environment has a catastrophic effect on the mechanical properties of the composites. Some form of oxidation protection is required in order to maintain structural capability. Much effort has been expended to develop technologies for preventing carbon oxidation. The following publications exemplify such technologies.
US 2007/0218208 A1, entitled BI- OR TRI-LAYER ANTI-OXIDATION SYSTEM FOR CARBON COMPOSITE BRAKES, describes a technology that provides a method of protecting a carbon-carbon composite brake disc or a carbon-carbon-silicon carbide composite brake disc against oxidation. The method includes: coating the composite brake disc with a first phosphoric acid-based penetrant system; curing the penetrant coating at 200° C. or above to form a first coating on the disc; applying a ceramic coating over the first coating and curing the ceramic coating at a temperature below 200° C. to form a second coating on the disc; and optionally coating the coated composite brake disc so obtained with a second phosphoric acid-based penetrant system, which would be cured at 200° C. or above to form a third coating.
US 2007/0154712 A1, entitled OXIDATION INHIBITION OF CARBON-CARBON COMPOSITES, describes a technology in which a barrier coating may be applied to a surface of a carbon-carbon composite prior to or subsequent to treatment with an oxidation inhibiting composition and/or pretreating composition. The barrier coating materials include carbides or nitrides, such as boron nitride, silicon carbide, titanium carbide, boron carbide, silicon oxycarbide, silicon nitride, and mixtures thereof.
US 2007/0199626 A1, entitled PROTECTION AGAINST THE OXIDATION OF COMPOSITE MATERIAL PARTS CONTAINING CARBON AND PARTS THUS PROTECTED, describes a technology that provides a part made of a composite material containing carbon, having an open internal residual porosity. The part is protected against oxidation by applying an impregnating composition, said impregnating composition containing a metal phosphate and titanium diboride. Efficient protection against oxidation is thus obtained at temperatures of more than 1000° C., even in the presence of carbon oxidation catalysts and moist conditions.
WO 1997/042135 A1, entitled IMPROVED OXIDATION PROTECTION FOR CARBON/CARBON COMPOSITES AND GRAPHITES, discloses a technology which improves oxidation resistance of carbon-carbon composites by application of colloidal silica to a surface of the composite and converting same to silicon carbide via reaction with the substrate during heat treatment.
Conventional antioxidant systems for carbon-carbon composite brake discs—including some of those discussed above—may fail due to temperature of use, runway de-icers, and humidity. In some cases, these factors cause the antioxidant to migrate within the carbon-carbon composite, which can lead to detrimental effects oxidation protection effectiveness and braking performance.